Interferometers are useful for non-contact measurement of small distance differentials such as the displacement or amplitude of a small vibration.
Typically, a two-beam interferometer includes a light source, typically a laser, that generates a coherent light having a wavelength λ. The light is split into two beams passing through two different optical paths. One of the beams is a probing beam which encounters and is reflected by a sample in its path and its optical path-length is dependent on the sample's position. The other beam is a reference beam which does not encounter the sample. The two beams are directed to re-converge at the end of their respective paths and the interference signal of the two beams is detected by a detector. The intensity of the interference signal is dependent on the phase difference in the two beams, and thus the optical path-length difference along the two paths. This, in turn, depends on the position of the sample. Thus, a change in the sample position causes a change in the intensity of the interference signal. Conversely, changes in the sample position can be extracted from the detected interference signal.
More specifically, the lack or presence of intensity changes can indicate whether the sample moves or vibrates along the path. The displacement of the sample is proportional to the changes in the amplitude of interference signal intensity.
In theory, the intensity of the interference signal is at a maximum or minimum when the path difference equals a multiple of λ/2 (δx=nλ/2), where n is an integer. Changes in the intensity of the interference signal are most sensitive to changes in δx when the path difference equals an odd multiple of λ/4 (δx=(2n+1)λ/4). Changes in intensity are also approximately linearly proportional to small changes in δx at or near a path difference of λ/4. As such, it is desirable to detect the interference signal at path difference near λ/4.
Some conventional interferometers include a path-length adjustor for adjusting the reference beam's path-length to keep δx proximate λ/4 during measurement. As the optical path-length can be affected by environmental factors, such as air flow in the beam path and temperature fluctuation, such a path-length adjuster may include a closed loop servo controlled actuator which moves a mirror in the reference beam's path based on intensity feedback from the detector. Such feedback actuators have been used in some conventional Michelson and Mach-Zehnder interferometers.
The actuated mirror can also be used to calibrate the interferometers to determine the peak-to-peak intensity change when the sample is not vibrating or moving.
However, these conventional interferometers suffer a few drawbacks. For example, the closed loop servo feedback is complicated and expensive. Further, such interferometers require pre-calibration to determine the peak-to-peak intensity change and the initial λ/4 path difference. Moreover, calibration cannot be performed when the sample is vibrating and may not be accurate since the laser light intensity may change after calibration due to changes in the laser source or in the beam paths. These drawbacks limit the usefulness of the conventional interferometers equipped with an actuated mirror controlled by a feedback servo.
Therefore, an improved laser interferometer is needed to overcome one or more of the shortcomings of the conventional interferometers.